The present invention relates to a method for forming a ceramics film on the surface of a metal substrate through anode-spark discharge and more specifically to a method for co-depositing fine ceramic particles and/or specific fine particles with ceramics components dissolved in the bath on the surface of a metal substrate by performing the spark discharge in a bath comprising a suspension containing these particles.
Ceramic films formed through an anode-spark discharge technique exhibit various excellent properties such as electrical insulating properties, low outgassing properties under ultra-high vacuum, corrosion resistance, flexibility and adhesion and, therefore, the spark discharge technique has become a center of attention as a technique for forming films.
Under such circumstances, there have been a variety of patents which relate to techniques for forming films by use of the spark discharge. For instance, U.S. Pat. Nos. 3,822,293; 3,834,999 and 4,082,626 disclose methods for forming films which comprise dissolving an alkali metal silicate or an alkali metal hydroxide or a combination of such an alkali with an oxyacid catalyst in water and performing spark discharge in the aqueous solution. In addition, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J. P. KOKOKU") No. Sho 58-17278 discloses a method for forming a film by use of an electric current having a specific wave form, which makes it possible to form a protective film on the surface of an aluminum substrate in an efficiency higher than that achieved by the foregoing methods disclosed in the U.S. Patents J. P. KOKOKU Nos. Sho 59-28636 and Sho 59-45722 also disclose methods for forming a colored protective film having a variety of color tones on an aluminum substrate, in which a metal salt or the like is added to an electrolytic bath.
On the other hand, J. P. KOKOKU No. Sho 59-28637 discloses a method for effectively forming a film on a magnesium or alloy substrate by use of an electric current having a specific wave form and J. P. KOKOKU No. Sho 59-28638 discloses a method for forming a protective film having a variety of color tones.
The foregoing methods disclosed in the aforementioned patents make it possible to form films having the foregoing characteristics, but the resulting films have low hardness, insufficient dielectric breakdown voltage and low film-forming velocity depending on the kinds of the electrolytic bath. In other words, these methods are not practical.